In recent years, many new satellite based services for vehicular (cars, airplanes . . . ) have come into service. These services include many applications such as satellite communications or global positioning systems. Compact antennas, generally arranged on the top of the vehicle, are required to receive these kinds of services together with traffic and emergency or security information data. These services are not only likely to be operated at different frequencies but also the radiation pattern requirements from the antenna will be different. For example, telecommunications may be provided via geostationary satellite system requiring antenna beams pointing at elevation between 20° and 60° at European latitudes while global positioning system requires antenna beams at zenith elevation.
The development of effective vehicular front-ends requires antennas with high directivity in the desired elevation angle, flat profile, lightweight, low-cost, and preferably conformable on curved surfaces.
A solution consisting in using an omnidirectional antenna should not be envisaged due to low gain. Another solution consisting in using a phase array for tracking satellites should also not be envisaged as being too expensive for standard consumer terminals. Printed antennas are incontestably the best suited kind of antennas for the development of such front-ends circuits of an antenna for vehicular mobile applications.
The requirements for user terminal antennas are tightly dependent on the associated space segment. Several existing and foreseen services will be based on geostationary space segment, which requires user segment antennas with intermediate gain (2-3 to 6-7 dBi). Typical user segment antennas for such applications can be subdivided in two main subsets: low and high latitudes. Low latitudes applications require antenna with a wide beam pointing in the vertical direction and their design does not present particular difficulties. At high latitudes, geostationary satellites are seen at an elevation angle between 66° down to 22°. In this case, user antennas for mobile applications must have the maximum directivity at an elevation angle of approximately 45° and they must be omnidirectional in azimuth. In other words, these user antennas must have a conical radiation pattern.
Printed antennas generating a conical radiation pattern are very interesting for the design of flat user terminal antennas for mobile satellite systems. Circular and annular patches resonating at higher modes are typical candidates to obtain such radiation patterns.
A prior art solution is disclosed in the U.S. Patent Application No. 2003/0210193. This document relates to a low-profile disk-shaped two-antenna assembly 100, shown on FIG. 11, including a first circular polarization ring antenna and a second linear monopole antenna that is located concentrically within the ring antenna. The antenna assembly 100 occupies then a cylindrical volume having a central axis.
The ring antenna comprises a metal resonant ring 101 tuned for the second-order mode (TM21) of operation, which is fed by a metal feed post 103 and its series-connected capacitor 104. The ring antenna is dielectrically loaded to reduce its physical size by positioning a low-dielectric plastic or dielectric ring 107 under resonant ring 101. The monopole antenna comprises two metal posts 105 spaced on opposite sides of the central axis and supporting at their top end a metal disk 106. Mechanical support for feed post 103, metal monopole posts 105 and for a metal ground plane 109 is provided by a PCB 108.
Both the ring antenna and the monopole antenna radiate in a conical radiation pattern, with the axis of the conical pattern extending generally perpendicular to the planar top surface of the antenna assembly 100 that contains both metal resonant ring 101 and metal disk 106.
However, U.S. Patent Application No. 2003/0210193 presents some drawbacks. Firstly, as it has been mentioned before, one of the most important requirement for user terminal antennas for mobile satellite communications is an antenna having a conical radiation pattern in the desired elevation angle, i.e. for instance between 20° and 60°, centered in the desired zone, for instance about 40-45°. In the antenna assembly presented in U.S. Patent Application No. 2003/0210193, both the ring antenna and the monopole antenna are excited via metal feed posts 103 and 105 which extend between the ground plane 109 and the corresponding radiating element 101 and 106.
It has been shown within the scope of the present invention, that such metallic feeding posts introduce perturbation into the conical radiation pattern. The resulting pattern is less homogenous than the theorical expected one and moreover the radiation amplitude is reduced. Therefore, the resulting antenna is less efficient.
Furthermore, with the goal of incorporating such an antenna assembly in a car-top application, the behaviour of this antenna assembly will be greatly influenced by the car-top material depending on whether it is glass, metal or plastic and also by the car-top design depending on whether it is plane, curved or with any fancy shape. Because the antenna disclosed in U.S. Patent Application No. 2003/0210193 is ground-plane dependent, the antenna radiation pattern has to be adjusted by using a metal pedestal.